Ion Nitriding Method

ABSTRACT

When a metal material such as an Fe alloy or Ni alloy is heated in the presence of an amino resin such as a melamine-formalin resin, a passivation film on the surface of the metal material is reduced and removed. During the reduction and removal of the passivation film, hydrogen sputtering may be also performed when the temperature becomes higher than 400° C., preferably not less than 450° C. By performing a glow discharge, the metal material having an exposed active metal surface is nitrided with N liberated from the amino resin. For example, the amino resin is applied onto the surface of the metal material directly or by using a solvent such as water.

TECHNICAL FIELD

The present invention relates to an ion nitriding method for introducing and diffusing nitrogen in a surface of a metal material under glow discharge.

BACKGROUND ART

A nitriding treatment, which is a surface modification treatment for introducing and diffusing nitrogen in a surface of a metal material to form a hard nitride, has been widely used with a purpose of hardening the surface to improve its abrasion resistance. This nitriding treatment is generally used for a material mainly composed of a Group 8 metal of the Periodic Table, e.g. an iron group alloy such as a stainless steel.

On the surface of the iron group alloy, a passivation film of an iron oxide is spontaneously formed due to oxidation by atmospheric oxygen. The passivation film inhibits the nitriding process, resulting in decreased nitriding efficiency.

Thus, generally a treatment for removing the passivation film is carried out before the nitriding treatment. As the removing treatment, a wet method of soaking the stainless steel in an aqueous cyanide solution, etc. has been conventionally used. However, the wet method is disadvantageous in that the cyanide is toxic, whereby the workers are burdened and a removing mechanism has to be installed in the workplace. Further, in this method, an apparatus for treating a waste liquid is required, thereby increasing equipment investment.

In view of solving such problems, a method for removing the passivation film by sputtering is proposed in Patent Document 1 and Non-Patent Document 1. It is reported in Patent Document 1 and Non-Patent Document 1 that a mixed gas of hydrogen with nitrogen or ammonia is introduced into a treatment furnace, glow discharge is caused in the treatment furnace to generate hydrogen and ammonia ions, and the passivation film is reduced and removed by the ions. A passivation film-reducing mechanism and a nitride-diffusing mechanism are both described in Non-Patent Document 1.

Further, a method proposed in Patent Document 2 contains the steps of introducing only a hydrogen gas into a treatment furnace at a temperature lower than 350° C., preferably lower than 150° C., and starting hydrogen gas sputtering at a relatively low temperature. It is described in Patent Document 2 that the passivation film can be reduced from inside such that H⁺ is diffused and introduced inside an iron group alloy base material, and the material is heated to a temperature equal to or higher than the reduction temperature of the passivation film, to transfer H⁺ to the surface of the material.

Patent Document 1: Japanese Patent Publication No. 02-002945 Patent Document 2: WO 01/34867

Non-Patent Document 1: Takao Takase, Ion Nitriding Method, Appearance of Rival of Soft Nitriding Method, KINZOKU, Separate print of March issue, 1973, Page 48 to 54

DISCLOSURE OF THE INVENTION

When a passivation film is reduced and removed from a steel having a relatively high Cr or Ni content by the method described in Patent Document 1 or Non-Patent Document 1, the passivation film cannot be sufficiently removed due to a small diffusion depth of H⁺ or NH₄ ⁺ in some cases. In such cases, disadvantageously, the thickness of a compound layer or a nitrided layer formed by the nitriding treatment is not uniform, or the compound layer or nitrided layer is formed on only a part of the steel.

In the method described in the Patent Document 2, only the diffusion and introduction of hydrogen ion into the iron group alloy base material is caused, the passivation film being not reduced. The characteristics of the diffusion and introduction of hydrogen ion, such as the diffusion depth, amount, and distribution of hydrogen ion, are greatly changed depending on sputtering conditions. When the hydrogen ion is diffused and introduced into an excessively deep position, or when the amount of the introduced hydrogen ion is too large, the hydrogen ion often causes hydrogen embrittlement. This tendency is particularly noticeable in a complicated member of an internal-combustion engine, etc.

Thus, for example, the hydrogen ion is diffused and introduced in the desired state by highly precisely controlling the sputtering condition, to prevent the so-called hydrogen embrittlement of the iron group alloy base material by remaining hydrogen ion. However, to perform such highly precise control, it is necessary to obtain data by repeating many tests complicatedly.

A general object of the present invention is to provide an ion nitriding method that can be carried out under secure environment easily and readily.

A principal object of the present invention is to provide an ion nitriding method that can be carried out with reduced material procurement costs.

Another object of the present invention is to provide an ion nitriding method capable of producing a compound layer and a nitrided layer that have a large thickness and are substantially uniform in any portion.

A further object of the present invention is to provide an ion nitriding method capable of preventing embrittlement of a nitriding-treated metal material.

According to an aspect of the present invention, there is provided an ion nitriding method comprising applying electricity to a metal material, thereby generating glow discharge, to ion-nitride the metal material in the presence of an amino resin.

When the metal material is heated in the presence of the amino resin, the amino resin is thermally decomposed and C, N, and H are liberated therefrom. The liberated C, N, and H interact with O to produce HCN or NO, which attacks and removes a passivation film, and eventually the passivation film is vanished. Thus, in the present invention, the substantially entire passivation film on the surface of the metal material can be easily removed by the remarkably simple process of heating the metal material in the presence of the amino resin.

Further, the amino resin is nontoxic according to Material Safety Data Sheet, and therefore the process can be carried out under secure environment. It should be noted that the above HCN is produced only in a slight amount of several-thousand ppm and is rapidly decomposed into nitrogen and carbon dioxide gas in waste gas combustion, so that an HCN-removing apparatus is not required.

The removal of the passivation film proceeds also during the heating process, and thereby temperature holding processes for the removal of the passivation film are not required in the present invention. Thus, due to the removal of the passivation film, the period for completing the ion nitriding treatment is not made longer, and the efficiency of the ion nitriding treatment is not reduced.

The amino resin is thermally decomposed into a gas phase, and exists as an ambient gas around the metal material. The gas phase contains the liberated N and acts as an N source for nitriding the metal material.

As compared with metal materials ion-nitrided without the amino resin, the metal material ion-nitrided in the presence of the amino resin has a higher hardness, and the hard region thereof extends more deeply. In the present invention, the ion nitriding treatment can be carried out without precisely controlling the types and ratios of gases, reaction temperature, reaction time, etc. as long as it is carried out in the presence of the amino resin. Various metal materials can be hardened by the method of the present invention.

Thus, as compared with ion nitriding processes without the amino resin, the simple easy process of nitriding in the presence of the amino resin according to the present invention can increase the hardness of the metal material higher. Further, the hardened region of the metal material extends more deeply in the present invention.

The metal material with the increased hardness is excellent in abrasion resistance and strength. Thus, an abrasion-resistant, high-strength metal material can be obtained in the present invention.

For example, the amino resin may be applied to the surface of the metal material. In this case, it is preferred that the amino resin is applied to the surface of the metal material using a solvent, so that application unevenness is reduced, and the passivation film removal and the nitriding proceed substantially uniformly in any portion.

The amino resin may be placed in a heat treatment furnace together with the metal material and then heat-treated without the application.

The amino resin is a resin produced by polycondensation between an amino group and formaldehyde. Typical examples of such resins include melamine resins, urea resins, aniline resins, and formalin resins.

Among these resins, the melamine resins are particularly preferred, and the melamine resin preferably contains a repeating unit having a composition formula of C₆H₃N₉. In this case, the residue of the resin is hardly attached to the heat treatment furnace during the ion nitriding treatment, whereby the number of maintenance can be significantly reduced, and the maintenance work can be easily carried out.

Preferred examples of the metal materials to be ion-nitrided include alloys mainly composed of Group 8 elements of the Periodic Table of Elements, such as Fe alloys and Ni alloys.

It is preferred that the heat treatment furnace is used as an anode and the metal material is used as a cathode to apply the electricity. In this case, it is not necessary to place another electrode in the heat treatment furnace, resulting in a simple structure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic, overall structural view showing an intake engine valve used in an ion nitriding method according to an embodiment of the present invention;

FIG. 2 is a graph showing an example of a temperature pattern of the ion nitriding method;

FIG. 3 is a graph showing an example of a temperature pattern of the ion nitriding method, containing an ion nitriding treatment with a nitriding gas;

FIG. 4 is a graph showing an example of a temperature pattern of the ion nitriding method, containing a hydrogen gas sputtering treatment and an ion nitriding treatment;

FIG. 5 is a schematic, overall structural view showing an exhaust engine valve used in an ion nitriding method according to another embodiment of the present invention;

FIG. 6 is a table showing the thickness and Hv of nitrided layers formed by a plasma ion nitriding treatment; and

FIG. 7 is a table showing the materials of engine valves, and the thicknesses and surface hardnesses of compound layers or nitride layers formed on the valves by a gas soft nitriding treatment using a melamine resin, in contrast to those obtained by not using the melamine resin.

BEST MODE FOR CARRYING OUT THE INVENTION

A preferred embodiment of the ion nitriding method of the present invention will be described in detail below with reference to the drawings.

An ion nitriding method according to an embodiment contains a first step of applying an amino resin to a surface of an intake engine valve 10 (a metal member) shown in FIG. 1, and a second step of ion-nitriding the intake engine valve 10 with the amino resin applied.

The intake engine valve 10 is installed in an internal-combustion engine of an automobile and reciprocally moved. The intake engine valve 10 has an integral structure of a wide head 12, a long bar 14, and an end 16, and is composed of SUH11M. Thus, the intake engine valve 10 is a Cr-containing steel material, i.e. a Cr-containing Fe alloy.

In the surface of the intake engine valve 10, the Fe alloy SUH11M is oxidized by atmospheric oxygen, whereby a passivation film of an oxide is spontaneously formed. Therefore, the amino resin is applied onto the passivation film in the first step.

The amino resin is a resin produced by polycondensation between an amino group (—NH₂) and formaldehyde, and typical examples thereof include melamine resins represented by the following structural formula (1), urea resins represented by the following structural formula (2), aniline resins represented by the following structural formula (3), and formalin resins. These amino resins are commercially available in the state of solid or powder.

The amino resin is particularly preferably a melamine-formalin resin containing a repeating unit having a composition formula of C₆H₃N₉. C₆H₃N₉ is a substance used specifically for salt bath nitriding. When the ion nitriding treatment is carried out using C₆H₃N₉, the residue of the resin is hardly attached to inner walls of a heat treatment furnace advantageously.

In the first step, it is preferred that a suspension liquid prepared by dispersing a powder of the amino resin in a solvent such as water is applied to the surface of the metal material, though the amino resin powder may be applied directly to the surface. In this case, application unevenness can be reduced, so that the thickness of the formed compound layer or nitrided layer, etc. can be substantially uniform.

The application may be achieved by a brush coating method using a brush. Of course, known application technologies other than the brush coating method may be used.

After the amino resin is applied directly or preferably by using the suspension liquid, the intake engine valve 10 and a heat treatment furnace are electrically connected to an electric power source. In this process, they are connected such that a furnace wall of the heat treatment furnace acts as an anode and the intake engine valve 10 acts as a cathode.

Then, the intake engine valve 10 is placed in the heat treatment furnace, and the heat treatment furnace is sealed to start the second step. A procedure of vacuuming the inside of the heat treatment furnace is started, while a procedure of rising the temperature of the furnace is started. The temperature rising rate may be 3° C. to 5° C./minute.

In the procedure of rising the temperature of the heat treatment furnace, the amino resin begins to be decomposed. As a result, C, N, and H contained in the amino resin are liberated therefrom to generate HCN. In a case where O interacts therewith, NO is also generated. The passivation film is reduced by attack of HCN or NO, and is removed finally. The active metal surface of the intake engine valve 10 is exposed in this manner.

As described above, according to this embodiment, the substantially entire passivation film on the surface of the intake engine valve 10 (the metal material) can be easily removed by the remarkably simple process of heating the intake engine valve 10 coated with the amino resin. Further, in this method, an existing apparatus such as the heat treatment furnace can be utilized, whereby particular additional equipment investment is not required.

It should be noted that the above HCN is produced only in an amount of several-thousand ppm, and is burned as the temperature of the heat treatment furnace rises. Thus, it is not necessary to set up an HCN-removing apparatus. Further, pretreatments such as treatments for removing an oxide film by using hydrofluoric acid, etc. are not required in this embodiment, and the amino resin is nontoxic, whereby the method can be carried out under secure environment.

As shown in a temperature pattern of FIG. 2, after the heat treatment furnace is heated to a predetermined temperature, electricity is applied to the intake engine valve 10 and the heat treatment furnace. The current and voltage may be about 25 A and about 220 to 250 V, respectively. When the electricity is applied, glow discharge is generated between the anode of the furnace wall of the heat treatment furnace and the cathode of the intake engine valve 10.

When the glow discharge is generated, the N liberated from the amino resin attacks the exposed metal surface of the intake engine valve 10. As a result, the liberated N is introduced and diffused in the metal surface, to nitride the surface.

Since the passivation film has been removed from the intake engine valve 10, the N does not have to penetrate the passivation film. Therefore, the time required for the nitriding treatment can be shortened, and the thermal energy can be reduced.

Further, since the passivation film can be removed in the temperature rising procedure during the nitriding treatment in this embodiment, the method can be carried out without a particular heat treatment process such as a treatment of keeping the metal material at a constant temperature to remove the passivation film. Thus, the nitriding treatment efficiency is not reduced due to the removal of the passivation film by the amino resin.

When the N is introduced and diffused inside the surface of the intake engine valve 10, the compound layer or the nitrided layer is formed. The thickness of the nitrided layer or the compound layer, i.e. the distance of the nitrogen diffusion in the intake engine valve 10, is significantly larger as compared with the case of a gas soft nitriding treatment carried out under the same conditions except for the absence of the amino resin. In the case of removing the passivation film under the presence of the amino resin and then ion-nitriding the metal material, the thickness of the compound layer or the nitrided layer can be increased, so that even a deeper region of the intake engine valve 10 can be hardened.

Specifically, in the case of using the intake engine valve 10 of SUH11M, the compound layer formed by the method using the melamine-formalin resin according to this embodiment has a thickness of approximately 81 μm, though the compound layer formed by the conventional gas soft nitriding treatment has a thickness of approximately 40 μm. Thus, the resultant intake engine valve 10 obtained by the treatment of this embodiment has a high hardness in a deeper region, as compared with the conventional gas soft nitriding treatment.

After the ion nitriding treatment of this embodiment, a fine martensite formed on the intake engine valve 10 can be observed by an electron beam probe microanalyzer (EPMA).

In the embodiment, as described above, the amino resin is applied to the surface of the intake engine valve 10 directly or by using a solvent, and the intake engine valve 10 is then heated, whereby substantially all the passivation film on the intake engine valve 10 can be removed easily and readily. When the intake engine valve 10 is subjected to the ion nitriding treatment, the intake engine valve 10 is nitrided substantially uniformly. Thus, the thickness unevenness of the compound layer or nitrided layer and the generation of a portion without the compound layer or nitrided layer can be reduced, and the resultant intake engine valve 10 has the layer with a large thickness and has a high hardness even in the deeper portion.

Further, in this embodiment, the same agent can be used for removing the passivation film and ion-nitriding the metal material. Therefore, the material procurement cost can be reduced in the embodiment.

The procedure of rising the temperature of the furnace is continued. For example, when the temperature reaches 520° C., this temperature is maintained for 60 minutes. Also, the vacuuming procedure is continued to maintain the inner pressure of the furnace at approximately 0.7 to 1.5 Torr.

In the above steps, in the case of applying the resin containing the repeating unit having the composition formula of C₆H₃N₉ to the intake engine valve 10, the residue of the resin is hardly attached to the walls of the heat treatment furnace. Thus, the number of maintenance of the heat treatment furnace can be significantly reduced, and the maintenance work can be simply and easily carried out. Further, the thickness of the compound layer can be increased.

After the temperature holding process, the temperature is lowered to approximately 200° C., the heat treatment furnace is opened, and the intake engine valve 10 is taken out. The compound layer or the nitrided layer is formed on the surface of the intake engine valve 10 by the ion nitriding treatment. The surface of the intake engine valve 10 taken out is hard due to the compound layer or the nitrided layer.

As shown in FIG. 3, a mixed gas of ammonia gas and RX gas, etc. may be flowed into the furnace after the temperature reaches approximately 500° C., and the above temperature holding may be carried out under the flow. In this case, the ammonia gas is ionized to produce nitrogen ion, and the nitrogen ion collides with the surface of the intake engine valve 10, which the passivation film is reduced and removed from. The surface of the intake engine valve 10 is nitrided also by the gas to form the compound layer or the nitrided layer.

As shown in FIG. 4, after the start of the glow discharge, a procedure of introducing a hydrogen gas may be started at a point A higher than 400° C. shown in FIG. 4, and thereby the passivation film may be reduced and removed by the amino resin and also by hydrogen gas sputtering. For example, the inner pressure of the heat treatment furnace may be approximately 0.7 to 1.5 Torr.

The hydrogen gas is converted to the plasma state, H⁺ in the plasma collides with the surface of the intake engine valve 10 under an electric field. The H⁺ is introduced as a diffusive hydrogen ion into the passivation film on the surface of the intake engine valve 10.

The temperature is sufficient for reducing the passivation film, and the introduced diffusive hydrogen ion is rapidly reacted with the passivation film. Thus, the diffusive hydrogen ion attacks the passivation film to reduce and remove the film. H₂O is generated by the reduction of the passivation film, and is rapidly discharged to the outside of the system.

In the case of starting the hydrogen gas sputtering at a temperature of 350° C. or lower, the hydrogen ion is diffused and introduced in the intake engine valve 10, the passivation film being not reduced. In the case of starting the hydrogen gas sputtering at a temperature of higher than 350° C. and not higher than 400° C., the rate of the diffusion and introduction of the hydrogen ion is too high as compared with the rate of reducing the passivation film.

In this case, the characteristics of the diffusion and introduction of the hydrogen ion, such as the diffusion depth, amount, and distribution of the hydrogen ion, are greatly changed depending on sputtering conditions. For example, when the voltage is excessively high, the amount or depth of the diffused and introduced hydrogen ion is excessively increased. Thus, the sputtering conditions have to be highly precisely controlled to obtain desired diffusion and introduction characteristics of the hydrogen ion at a temperature of 400° C. or lower.

In contrast, in the case of starting the hydrogen gas sputtering at a temperature higher than 400° C., the diffusive hydrogen ion collides with the intake engine valve 10, is introduced in the passivation film, rapidly causes the reduction of the passivation film, and thus is rapidly consumed. Meanwhile the hydrogen gas is emitted from the inside of the intake engine valve 10, whereby the amount or depth of the diffused and introduced hydrogen ion is not excessively increased even when the voltage is excessively high. Thus, the reduction and removal of the passivation film can be easily controlled.

The hydrogen gas sputtering is preferably carried out at a temperature of 450° C. or higher. At the temperature, the reduction of the passivation film is remarkably accelerated while the hydrogen ion is emitted at a high rate from the intake engine valve 10. Thus, the passivation film can be reduced and removed without highly precisely controlling the sputtering conditions.

It is preferred that the hydrogen gas is supplied in addition to the nitriding gas also in the temperature holding process. In this case, the hydrogen gas sputtering proceeds along with the ion nitriding, so that reproduction of the passivation film on the surface of the intake engine valve 10 can be prevented. The volume ratio between the nitriding gas and the hydrogen gas may be, for example 2:1.

A second embodiment, which contains the steps of placing an amino resin in a container and introducing the container in a heat treatment furnace together with an exhaust engine valve 20 shown in FIG. 5, will be described below. In the second embodiment, the amino resin is not applied to a surface of the exhaust engine valve 20, and is placed in the powder state in the heat treatment furnace.

The exhaust engine valve 20 has a wide head 22, a long bar 24, and an end 26. The bar 24 is divided at a portion slightly closer to the head 22 than its center. Thus, the bar 24 has a first bar 28 and a second bar 30 that is slightly longer than the first bar 28.

The head 22 and the first bar 28 are composed of NCF600, and the second bar 30 and the end 26 are composed of SUH11M. In the surface of the exhaust engine valve 20, NCF600 and SUH11M are spontaneously oxidized, whereby a passivation film of an oxide is formed.

The exhaust engine valve 20 having such a structure is degreased using an organic solvent, etc., and is placed in the heat treatment furnace together with the above described amino resin. Then, the exhaust engine valve 20 and the heat treatment furnace are electrically connected to an electric power source such that the valve 20 acts as a cathode and a furnace wall of the furnace acts as an anode, to start electricity application. The current and voltage may be about 25 Å and about 220 to 250 V, respectively.

When the amino resin is placed in the heat treatment furnace, the amount of the amino resin may be about 1% to 10% of the weight 1 kg of the metal material. For example, when the weight of the exhaust engine valve 20 is 10 kg, 0.1 to 1 kg of the amino resin may be added to the container and placed in the heat treatment furnace.

Then, a compound layer and a nitride layer are formed on the exhaust engine valve 20 by carrying out an ion nitriding treatment in the manner shown in FIGS. 2 to 4.

The exhaust engine valve 20 may comprise a so-called superalloy such as 30Ni15Cr or 75Ni15Cr. For example, in the case of using 75Ni15Cr in the exhaust engine valve 20, a compound layer having a thickness of approximately 5 μm can be formed such that the exhaust engine valve 20 and a melamine resin are placed in the heat treatment furnace, the weight ratio of the melamine resin to the exhaust engine valve 20 being 5%, and they are kept at 540° C. for 2 hours under glow discharge to carry out the ion nitriding treatment.

The materials of the head 22 and the first bar 28 are not limited to NCF600, and may be NCF3015, NCF440, or the like. Also the materials of the second bar 30 and the end 26 are not limited to SUH11M, and may be SKH51 or the like. The thickness and Hv of nitrided layers, formed by subjecting NCF3015, NCF440, and SKH51 to the above hydrogen gas sputtering treatment and ion nitriding treatment, are shown in FIG. 6. In the case of using NCF600 or SUH11M, the nitriding treatment was started at 500° C., the temperature of the heat treatment furnace reached 520° C. 20 minutes after the start, and this temperature was kept for 40 minutes. On the other hand, in the case of using NCF3015 or SKH51, the nitriding treatment was started at 520° C., the temperature rise was stopped, and the temperature of 520° C. was kept for 60 minutes.

In the embodiment, the passivation film can be easily and readily removed from the surface of the various metal material to accelerate the nitriding treatment in this manner. Further, any material can be ion-nitrided in the presence of the amino resin without precisely controlling the types and ratios of gases, reaction temperature, reaction time, etc.

The temperature, kept in the ion nitriding treatment, is not limited to the above temperature of approximately 520° C. When the temperature is excessively high, the rate of the reproduction of the passivation film is too high as compared with the rate of the reduction and removal thereof, and the produced nitride is diffused in the intake engine valve 10 or the exhaust engine valve 20, so that the nitrided layer is hardly formed. To prevent the reproduction of the passivation film and the diffusion of the nitride, the temperature is preferably 590° C. or lower, more preferably 550° C. or lower, most preferably 520° C. to 540° C.

Though the iron group alloys and Ni alloys are used as the metal material in the above described embodiments, the metal material is not particularly limited thereto. The metal material may be any member composed of a metal alloy containing, as a main component, a Group 8 metal element of the Periodic Table, such as a Cr alloy. The conditions of the plasma nitriding treatment, such as the kept temperature and the treatment time, may be appropriately selected depending on the material of the work.

In the above embodiment, the electricity is applied to the intake engine valve 10 such that the furnace wall is used as an anode and the intake engine valve 10 is used as a cathode. For example, the electricity may be applied such that an electrode is installed in the heat treatment furnace, the electrode is used as an anode, and the intake engine valve 10 is used as a cathode.

Example 1

An engine valve was prepared from each of metal materials shown in FIG. 7, the engine valve and a melamine resin were placed in a heat treatment furnace, and the engine valve was ion-nitrided at 520° C. for 60 minutes. For comparison, the metal materials were ion-nitrided in the same manner except for not placing the melamine resin in the heat treatment furnace. The thickness of the compound layer or nitride layer and the surface hardness of each engine valve ion-nitrided in the presence of the melamine resin are shown in FIG. 5 in multiples of those of each engine valve ion-nitrided without the melamine resin. It is clear from FIG. 5 that the thickness of the compound layer or nitride layer and the surface hardness can be increased by using the melamine resin in the ion nitriding treatment. This means that, when the ion nitriding treatment is carried out in the presence of the melamine resin, a passivation film is removed and whereby the nitride is substantially uniformly diffused deep into the metal material.

INDUSTRIAL APPLICABILITY

In the present invention, the metal material is ion-nitrided in the presence of the amino resin. By the simple procedure, the passivation film on the surface of various metal material can be easily removed and the surface can be easily nitrided under secure work environment. Thus, the compound layer or nitrided layer having a substantially uniform thickness can be formed on the approximately entire surface of the metal material.

Further, in the present invention, the thickness of the compound layer or nitrided layer can be increased. In other words, the metal material can show improved hardness, strength, abrasion resistance, etc. even in a deep inside region. 

1. An ion nitriding method comprising applying electricity to a metal material, thereby generating glow discharge to carry out a nitriding treatment on said metal material, wherein said metal material is ion-nitrided in the presence of an amino resin in said nitriding treatment.
 2. An ion nitriding method according to claim 1, wherein said amino resin is applied to a surface of said metal material before said nitriding treatment.
 3. An ion nitriding method according to claim 2, wherein said amino resin is applied to said surface of said metal material using a solvent.
 4. An ion nitriding method according to claim 1, wherein said amino resin and said metal material are placed in a heat treatment furnace before said nitriding treatment.
 5. An ion nitriding method according to claim 1, wherein said amino resin is a melamine resin, a urea resin, an aniline resin, or a formalin resin.
 6. An ion nitriding method according to claim 5, wherein said melamine resin contains a repeating unit having a composition formula of C₆H₃N₉.
 7. An ion nitriding method according to claim 1, wherein a heat treatment furnace is used as an anode and said metal material is used as a cathode to apply said electricity. 